The Federal Aviation Administration came under sharp attack at a Senate hearing as industry officials and congressional investigators asserted that airline passengers' safety too often depends on poorly trained inspectors and a mismanaged computer system. One FAA inspector, who testified behind a screen out of fear of agency reprisal, said he was trained on commuter planes but was forced to inspect big jets. An ex-inspector with an airline, who also testified behind a screen, said he was ordered to hide violations.